Conventionally, thermal transfer method has been widely used as a simple printing method. The thermal transfer method is an image forming method wherein a thermal transfer sheet which is provided with a colorant layer formed on a surface of a substrate sheet thereof is superposed on a thermal transfer image-receiving sheet which is provided with a receiving layer, then the back side of the thermal transfer sheet is heated by a heating means such as a thermal head with a shape of an intended image, and thereby the colorant included in the colorant layer is selectively transferred to the thermal transfer image-receiving sheet to form the image on the receiving layer of the thermal transfer image-receiving sheet.
The thermal transfer method may be divided into two methods, i.e., melt-transfer method and sublimation transfer method. The melt-transfer method is an image forming method wherein a thermal transfer sheet, in which a thermally fusible ink layer comprising a colorant, such as a pigment, and a binder for dissolving or dispersing the colorant, such as a thermally fusible wax or resin, is carried on a substrate sheet, is used; then, an energy in accordance with image information is applied to the thermal transfer sheet by a heating means such as a thermal head; and thereby the colorant is transferred onto the thermal transfer image-receiving sheet, such as a plastic sheet or paper, together with the binder. The image formed by the melt-transfer method is suitable for recording binary images such as characters, since the image has excellent sharpness and high concentration.
On the other hand, the sublimation transfer method is another image forming method wherein a thermal transfer sheet, in which a colorant layer comprising a colorant which is able to be transferred thermally by sublimation, and a resin binder for dissolving or dispersing the colorant, is carried on a substrate sheet, is used; then, an energy in accordance with image information is applied to the thermal transfer sheet by a heating means such as a thermal head; and thereby only the colorant is transferred (diffusion-transferred) onto a substrate sheet, such as a plastic sheet or paper, or onto a thermal transfer image-receiving sheet which is optionally provided with a receiving layer. With respect to the sublimation transfer method, since the transferring amount of colorant can be controlled by the amount of energy applied, it is possible to form a gray-scale image in which image density is controlled. In addition, since the colorants used are dyes, the image formed has transparency. Thus, when dyes of different colors are superposed, the reproducibility of neutral tints becomes excellent. Therefore, when using thermal transfer sheets of different colors such as yellow, magenta, cyan, black or the like, and transferring dye of each color on the thermal transfer image-receiving sheet so as to superpose the dyes each other, it is possible to form a photographic full-color image of high quality in which reproducibility of neutral tints is excellent.
Along by the development of various hardware and software related to multimedia, this thermal transferring method has been expanding its market as a full-color hard copy system for digital images represented by computer graphics, satellite static images, CD-ROM or the like, and for analog images such as video. The thermal transfer image-receiving sheet according to the thermal transfer method includes a wide range of concrete applications. As typical examples, proof printing; image output; output of plan or design, such as those drawn by CAD/CAM, etc.; output use for various medical analytical or measuring instruments such as CT scan, endoscopic camera, etc.; and a substitute for instant photos; as well as, output of picture of face to an identification paper or ID card, credit card, or other cards; and applications as composite photograph or souvenir picture at an amusement facilities such as amusement park, amusement arcade, museum, and aquarium, etc., can be mentioned.
With the diversification of use of the above-mentioned thermal transfer image-receiving sheet, there is an increasing demand for forming a thermal transfer image on an arbitrary object. As the object for forming the thermal transfer image, a purpose-built thermal transfer image-receiving sheet which is provided with a receiving layer on the substrate is usually utilized. However, in this case, the substrate or the like subjects to some type of constraints. Under these circumstances, Patent literature 1 discloses an intermediate transfer medium in which the receiving layer is provided on the substrate so that the receiving layer can be peeled off from the substrate. According to the intermediate transfer medium, by transferring the colorant of the colorant layer included in a thermal transfer sheet to the receiving layer so as to form a thermal transfer image, and then heating the intermediate transfer medium, it is possible to transfer the receiving layer onto which the colorant has been transferred to an arbitrary transfer receiving article. Thus, it becomes possible to form a thermal transfer image without concern for the constraints about the kind of transfer receiving article.
A print having a high-density thermal transfer image is highly demanded these days, and in response to this demand, a method is used wherein the amount of colorant transferred from the colorant layer to the receiving layer of a thermal transfer sheet is large. The amount of colorant transferred from the colorant layer to the receiving layer of a thermal transfer sheet varies according to the amount of energy applied to the back face side of the thermal transfer sheet during the formation of a thermal transfer image, and the larger the amount of energy is, the larger the amount of colorant transferred onto the receiving layer is. Accordingly, it is the current situation that, in response to the above-mentioned demand, the amount of energy applied to the back face side of the thermal transfer sheet has to be increased to form a high-density thermal transfer image on the receiving layer.
When the amount of energy applied to the back face side of the thermal transfer sheet is increased to form a high-density thermal transfer image on the receiving layer, the receiving layer is thermally damaged. Formation of a print by using an intermediate transfer medium requires transfer of the receiving layer, on which a thermal transfer image has been formed, onto a transfer receiving article after formation of the thermal transfer image on the receiving layer as described above. Thus, if the receiving layer is thermally damaged during formation of the thermal transfer image, transferability when transferring the receiving layer, on which the thermal transfer image has been formed, onto a transfer receiving article is lowered, or in other words, the problem of lowered adhesion between the transfer receiving article and the receiving layer on which the thermal transfer image has been formed arises. In addition, due to the lowered adhesion between the transfer receiving article and the receiving layer on which the thermal transfer image has been formed, a solvent or the like is likely to enter through the interface between the transfer receiving article and the thermal transfer image, problems such as poor durability of the formed print are likely to arise as well.